shader_runner: Add support for layered framebuffer

This allows attaching an entire 2darray texture as a layered
framebuffer.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
shader_runner: Support creating uninitialized 2d array textures

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
arb_fragment_layer_viewport: Add tests for FS reading correct values

This covers both in-range and out-of-range cases.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>